This invention relates generally to regulating the flow of molten metal by electromagnetism and is particularly directed to controlling the flow of molten metal in an electromagnetic process for continuous casting of steel.
Steel making occupies a central economic role and represents a significant fraction of the energy consumption of many industrialized nations. The bulk of steel making operations involves the production of steel plate and sheet. Present steel mill practice typically produces thin steel sheets by pouring liquid steel into a mold, whereupon the liquid steel solidifies upon contact with the cold mold surface. The solidified steel leaves the mold either as an ingot or as a continuous thick slab after it is cooled typically by water circulating within the mold wall during a solidification process. In either case, the solid steel is relatively thick, e.g., 6 inches or greater, and must be subsequently processed to reduce the thickness to the desired value and to improve metallurgical properties. The mold formed steel is usually characterized by a surface roughened by defects, such as cold folds, liquation, hot tears and the like which result primarily from contact between the mold and the solidifying metallic shell. In addition, the steel ingot or slab thus cast also frequently exhibits considerable alloy segregation in its surface zone due to the initial cooling of the metal surface from the direct application of a coolant. Subsequent fabrication steps, such as rolling, extruding, forging and the like, usually require the scalping of the ingot or slab prior to working to remove both the surface defects as well as the alloy deficient zone adjacent to its surface. These additional steps, of course, increase the complexity and expense of steel production.
Steel slab thickness reduction is accomplished by a rolling mill which is very capital intensive and consumes large amounts of energy. The rolling process therefore contributes substantially to the cost of the steel sheet. In a typical installation, a 10 inch thick steel slab may be manipulated by ten rolling machines before it reaches its commercial thickness. The rolling mill may extend as much as one-half mile and cost as much as $500 million.
Another approach to forming thin metal sheets involves casting into approximately the final desired shape. Compared to current practice, a large reduction in steel sheet total cost and in the energy required for its production could be achieved if the sheets could be cast in near net shape, i.e. in shape and size closely approximating the final desired product. This would reduce the rolling mill operation and would result in a large savings in energy. There are several technologies currently under development which attempt to achieve these advantages by forming the steel sheets in the casting process. Some of the approaches use electromagnetic fields on one or both sides of the liquid metal sheet to confine the sheet as it solidifies in a continuous process.
Systems that employ magnetic fields as a substitute for a mechanical mold wall to confine a molten metal in a continuous casting process include U.S. Pat. No. 4,678,024, "Horizontal Electromagnetic Casting of Thin Metal Sheets", by Hull et al., issued July 7, 1987; U.S. Pat. No. 4,905,756, "Electromagnetic Confinement and Movement of Thin Sheets of Molten Metal" by Lari et al., issued Mar. 6, 1990; U.S. Pat. No. 4,936,374, "Sidewall Containment of Liquid Metal with Horizontal Alternating Magnetic Fields" by Praeg, issued June 26, 1990; and copending applications: "Sidewall Containment of Liquid Metal with Vertical Alternating Magnetic Fields" by Lari et al., Ser. No. 408, 418, Notice of Allowability dated June 15, 1990, continuation of Ser. No. 207,818, now abandoned; and "Electromagnetic Confinement for Vertical Casting or Containing Molten Metal" by Lari et al., Ser. No. 257,387, Notice of Allowability dated July 10, 1990.
These continuous casting processes include a feed system that continuously introduces metal in liquid form to the electromagnetic casting process. The feed system includes a tundish or reservoir for the molten metal from which the molten metal flows via a nozzle to the electromagnetic caster. The molten metal can flow from the tundish through the nozzle by gravity. As part of the electromagnetic casting processes, it can be necessary or advantageous to be able to precisely control the flow of molten metal from the tundish and to vary the flow rate to account for start-up or shut-down conditions. Also, control of the flow can be incorporated into a feedback system to exercise precise and accurate control over the casting process to insure a high quality product. To accomplish this it is necessary to control the flow of molten steel from the tundish within a narrow operating range and to be able to precisely modify it in response to feedback signals. The design for the feed system would include the following factors: (1) casting start-up transients must be closely controlled; (2) during casting, the fluid volume flow rate must equal the casting speed; (3 ) changes in ferrostatic pressure (p.sub.g =.rho.gh, where .rho.=fluid density, g=acceleration due to gravity, and h=effective vertical height of the molten steel) must not have a detrimental effect on the electromagnetic continuous caster; (4) the cross-section of the molten metal stream emerging from the feed system must be approximately that of the desired product (i.e., a rectangular sheet having a relatively large aspect ratio).
One of the reasons that a flow control system has such significance in the continuous casting process is that the weight of the molten steel exerts such high pressure in the casting system. For example, with steel the ferrostatic pressure of an effective height of only 5.1 cm can produce a casting speed of 1 ms.sup.-1. A height variation of only 0.1 cm (0.039 inches) results in a 1% speed variation. The speed of 1 ms.sup.-1 is reached in 0.1 second if the gate of the feed system is opened abruptly. Controlling the flow by mechanical means only (adjustable orifices, dams, or weirs) can be difficult especially if one has to compensate for the changing level of molten steel in the tundish and for build-up or erosion in the nozzle. Accordingly, there is a need for a control device that overcomes these problems associated with mechanical feed systems.
The present invention has particular application to use in a continuous casting system that uses electromagnetic fields to confine a molten metal as it solidifies. However, the present invention could also be used in continuous casting systems that do not employ electromagnetic fields to confine the molten metal. This invention can be utilized wherever it is necessary to carefully regulate the flow of molten metal.
Accordingly, an object of this invention is the production of a precisely controlled flow of molten metal into a caster during start-up, during continuous operation, during shut-off and during shut-down.
Another object of this invention is the precise control of molten metal in a continuous casting process in the presence of mechanical disturbances caused by start-up transients, metal being poured from the ladle into the tundish (e.g. turbulence, ripples, changes in flow pattern, etc.), or wear and tear on orifices, dams, or weirs used to mechanically control the metal flow.
A further object of this invention is to supplement a mechanical molten metal feed system with an electromagnetic system that controls the fluid flow with a traveling electromagnetic field.
A still further object of this invention is to control a mechanical molten metal feed system with an electromagnetic system that regulates the molten metal flow with a traveling electromagnetic field via feedback loops that include sensors responsive to the casting speed, ferrostatic pressure, casting sheet thickness, temperature or spatial location of a solid metal sheet as it emerges from the caster.
A yet still further object of this invention is the electromagnetic control of molten metal flow into a caster that can cast horizontally, vertically, or in a slanted direction by means of a traveling electromagnetic wave produced by a magnet arranged around a duct coupled to the nozzle of a tundish that feeds the caster.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.